No. 45: May-Jun 1986
Some of our continent's most powerful earthquakes have shaken the eastern half rather than the Pacific states, where the edges of tectonic plates grind together. The devastating New Madrid and Charleston quakes did not occur at plate boundaries, and it is hard to find active faults to blame for the crustal commotion. A hint of a possible solution to the dilemma comes with the correlation of earthquakes with heavy rainfalls and high water tables. For example, the Charleston quake of 1886 was preceded by two years of unusually heavy rainfall followed by a short dry spell. Also, seismicity in the New Madrid (MO) area increases 6-9 months after the Mississippi has crested. The theory is that the added water penetrates deep into the earth where it lubricates faults, causing them to become active and jolt the surface above.
(Weisburd, S.; "Trickle-Down Theory of Eastern Quakes," Science News, 129:165, 1986.)
Comment. The above correlations and our inability to explain deep-focus earthquakes underscore our ignorance of the mantle. To illustrate, Soviet drillers have found fluids circulating through fractured rocks 11 kilometers down, where one would expect every thing to be sealed tight by the weight of the overlying sediments.